The present invention relates to a mass spectroscope and a method for analyzing ions. More particularly, the present invention relates to a mass spectroscope having an ion retention portion between an ion source and a mass spectrometry portion for storing, cooling ions and for dissociating ions, and a method for analyzing the ions with such a mass spectroscope.
A conventional mass spectroscope has a configuration in which an ion source generates ions, and the ions are introduced to a mass spectrometer such as a quadrupole mass filter or time-of-flight mass spectrometer, so that the ions are separated according to each mass number (mass/charge) and detected. Recently, a mass spectroscope has been developed in which an ion trap, as an ion retention portion, is provided between an ion source and a mass spectrometer. The ion retention portion accumulates and stores various types of ions generated from the ion source. Then, the ions are accordingly sorted, or released from the ion retention portion, and are introduced to the mass spectrometer. The ion trap may have a function of sorting ions according to the mass number. Therefore, a mass spectrometer at a later stage may be used only as a detector, and the ion trap separates ions according to the mass number and directly introduces the ions to the detector.
In the ion trap, a voltage is applied to multiple electrodes constituting the ion trap to form a quadrupole electric field for trapping ions and storing the ions in an ion trapping space. In a mass spectroscope with such an ion trap, it is difficult to obtain sufficient trapping efficiency only with the electric field. Therefore, a gas called a cooling gas may be introduced into the ion trap so that the ions collide with gas molecules. Accordingly, the ions change a direction of motion thereof and converge their trajectories toward the center of the ion trap (for example, refer to Japanese Patent Publication No. 09-189681). This is called a cooling operation because a kinetic energy of the ion is reduced by collisions.
In a case that the ions are dissociated for analyzing fragment ions while the ions are stored in the ion trap, a gas is introduced into the ion trap for inducing collision-induced dissociation, so that the ions collide with the gas molecules and dissociate into fragment ions.
Japanese Patent Publication No. 2002-184349 has disclosed a mass spectroscope in which an additional ion retention portion in front of an ion trap is provided. In the mass spectroscope, a cooling gas is supplied in the ion retention portion before ions are introduced into an ion trap, so that the ions are efficiently introduced into the ion trap. In this case, the cooling gas molecules colliding with the ions absorb ions' kinetic energy as described above.
In the mass spectroscope described above, the cooling gas or the dissociation gas is introduced to the ion retention portion, so that the ions collide with the gas molecules to control the trajectories thereof or enhance efficiency of dissociation of the ions. In this case, when a mass spectrum is obtained through scanning the mass number, the mass spectrum does not have good peak separation.
In view of the problem, the present invention has been made, and an objective of the present invention is to provide a mass spectroscope in which a cooling gas or dissociation gas is introduced in a controlled manner. With the mass spectroscope, it is possible to obtain excellent peak separation when the ion mass is scanned, so that an analysis can be performed with a high mass resolution.
Further objects and advantages of the invention will be apparent from the following description of the invention.